A surveillance camera has generally two functions which are a day mode to capture an image at daytime and a night mode to capture an image at nighttime. The day mode is a function to capture a normal color image. On the other hand, in the night mode, in order to capture an image in a dark environment at nighttime, infrared light (infrared ray) is projected and the reflected light thereof is captured. In this manner, even in environment with no visible light, it is possible to acquire a clear image (hereinafter referred to as an infrared image).
However, in capturing image by infrared light unlike visible light, color information cannot be acquired. Therefore, the image is generally displayed on a display device in a monochrome of gray or green based on the brightness of infrared light.
On the other hand, a surveillance camera is used to monitor a suspicious person or a suspicious object in a surveillance area. In order to identify them, the color information, such as colors of person's clothes and a vehicle, is extremely important. However, when the image is captured by the normal color mode at dark time, such as nighttime, noise and the signal intensity of an object are at the same level and it is difficult to distinguish them.
To solve these problems, the above mentioned night mode by infrared light irradiation is used. However, the image obtained by infrared light irradiation is a monochrome image in which the color of the object cannot be distinguished, whereas the image is as clear as that captured at daytime.
Furthermore, in addition to a surveillance camera, a digital video camera, a camcorder and the like have the function to capture an image at dark time by infrared light irradiation. In such devices, it is required to color the infrared image to obtain a natural image.
With regard to the above described problems, Patent Document 1, for example, discloses a technique for adding colors to the infrared image when there is no visible light due to environment. With the technique, three types of infrared light having a different wavelength is used as infrared light to be projected on an object to estimate the color of the object based on the difference (correlation) between reflection characteristics of infrared light by a substance (resin) and reflection characteristics of visible light by the substance. However, with this technique, by using a plurality of light sources and spectral systems thereof, it is likely to increase in cost and the color estimation accuracy is not sufficiently high.
On the other hand, when visible light which slightly remains in environment is used, it is possible to perform color reproduction to some extent with a conventional camera system by devising a method of a signal process. As an example of the technique, in a digital camera, a technique for reproducing natural colors on an image captured by projecting infrared light at dark time (night shot) is proposed (for example, see Patent Document 2). In the technique, when the camera detects that the camera system is changed to a night mode, a parameter table different from the one used in a normal color image capturing mode is used. The parameter table is used for a white balance adjustment. An appropriate color reproduction is thereby performed even when visible light and infrared light are mixed.